Synthesis of pyrochlore nanostructures and uses thereof

ABSTRACT

A template-free reverse micelle (RM) based method is used to synthesize pyrochlore nanostructures having photocatalytic activity. In one embodiment, the method includes separately mixing together a first acid stabilized aqueous solution including pyrochlore precursor A and a second acid stabilized aqueous solution including pyrochlore precursor B with an organic solution including a surfactant to form an oil-in-water emulsion. Next, equimolar solutions of the first and second acid stabilized oil-in-water emulsions are mixed together. Then, the mixture of the first and second acid stabilized oil-in-water emulsion is treated with a base to produce a precipitate including pyrochlore precursors A and B. After which, the precipitate is dried to remove volatiles. The precipitate is then calcined in the presence of oxygen to form a pyrochlore nanostructure, such as a bismuth titanate (Bi 2 Ti 2 O 7 ) pyrochlore nanorod. The method of synthesizing the pyrochlore nanorod is template-free.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/333,463, filed May 11, 2010, which is hereby incorporated byreference herein in its entirety.

TECHNICAL FIELD

The present invention relates generally to pyrochlore nanostructuresand, more specifically, to synthesis of pyrochlore nanorods, such asbismuth titanate pyrochlore nanorods, and uses thereof.

BACKGROUND

Photocatalytic hydrogen production is a possible answer to the presentenergy crisis. However, efficient hydrogen production requires thedevelopment of materials that demonstrate tunable optical response, haveUV-visible activity, and are amenable to synthesis using simple methods.Metal oxides, mainly titanium dioxide (TiO₂), have been studied forphotocatalytic applications. Fujishima et al., Nature 1972, 238, 37;Hoffmann et al., Chem. Rev. 1995, 95, 69. However, decades of work withTiO₂, such as to improve its properties via doping and addition of otherphotoactive materials, has demonstrated only incremental improvements.Martyanov et al., Chem. Commun., 2004, 2476; Anpo et al., J. Catal.,2003, 216, 505.

Pyrochlores (A₂B₂O₇) are a lesser studied family of compounds that offerthe flexibility to tune photocatalytic properties. Bismuth titanate(BTO), Bi₂Ti₂O₇, is a photoactive member of the pyrochlore family thatcan potentially meet the aforementioned objectives desired of aphotocatalyst. However, the full potential of this photocatalyst forhydrogen generation has not yet been fully exploited, which is partiallybecause attempts to synthesize and characterize stoichiometric BTOpyrochlore are far and few. The earlier adopted synthesis methods havenot always resulted in a pure pyrochlore phase. Yao et al., Appl. Catal.B Environ. 2004, 52, 109; Yao et al., Appl. Catal. A. Gen. 2003, 243,185; Yao et al., J. Mol. Catal. A. 2003, 198, 343. For example,Radosavljevic et al. reported a method that resulted in a Bi—Ti—Ocomposition of the form Bi_(1.74)Ti₂O_(6.62). Radosavljevic et al., J.Solid State Chem. 1998, 136, 63. Hector and Wiggin reported thesynthesis and structural study of stoichiometric BTO along with someimpurities of Bi₄Ti₃O₁₂ using a co-precipitation method. Hector et al.,J. Solid State Chem. 2004, 177, 139.

Recently, attempts to prepare stable crystalline BTO with a nanotubestructure using anodized alumina as a template have been reported. Zhouet al., J. Mater. Res. 2006, 21, 2941. A template method, however,requires template removal, which is an additional step. To synthesizethese materials economically on a large scale requires the developmentof a template-free method.

Accordingly, a need exists for new template-free methods of synthesizingpyrochlore nanostructures, such as pyrochlore nanorods.

SUMMARY OF THE INVENTION

According to one embodiment of the invention, a method of synthesizing apyrochlore nanostructure having the general formula A₂B₂O₇, where A andB are rare-earth or transition metal species and O is oxygen, isprovided. The method includes separately mixing together a first acidstabilized aqueous solution including a pyrochlore precursor A and asecond acid stabilized aqueous solution including a pyrochlore precursorB with an organic solvent mixture including a surfactant and an organicsolvent to form a first acid stabilized oil-in-water emulsion ofprecursor A and a second acid stabilized oil-in-water emulsion ofprecursor B. Next, equimolar portions of the first and second acidstabilized oil-in-water emulsions are mixed together to form a mixedacid stabilized oil-in-water emulsion including the pyrochlore precursorA and the pyrochlore precursor B. Then, the mixed acid stabilizedoil-in-water emulsion is treated with a base to increase the pH fromacidic to alkaline to produce a precipitate including the pyrochloreprecursors A and B. The precipitate is isolated then calcined in thepresence of oxygen to form the pyrochlore nanostructure, wherein themethod of synthesizing the pyrochlore nanostructure is template-free.

According to another embodiment of the invention, a pyrochlorenanostructure having the general formula A₂B₂O₇ is provided wherein A isbismuth (Bi), calcium (Ca), strontium (Sr), yttrium (Y), barium (Ba),lanthanum (La), or combinations thereof; B is titanium (Ti), vanadium(V), chromium (Cr), manganese (Mn), iron (Fe), nickel (Ni), zirconium(Zr), tin (Sn), hafnium (Hf), tantalum (Ta), tungsten (W), orcombinations thereof; and O is oxygen; and wherein the nanostructure hasa spherical or rod shape.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with a general description of the invention given above, andthe detailed description of the embodiments given below, serve toexplain the principles of the invention.

FIG. 1A is a schematic representation of a reverse micelle (RM) basedtemplate-free method for synthesizing pyrochlore nanostructures inaccordance with an embodiment of the invention;

FIG. 1B is a schematic representation of a reverse micelle (RM) basedtemplate-free method for synthesizing bismuth titanate pyrochlorenanorods in accordance with an embodiment of the invention;

FIG. 2A is a scanning electron microscope (SEM) image of bismuthtitanate (BTO) calcined at 650° C.;

FIG. 2B is an x-ray powder diffraction (XRD) pattern of the BTO;

FIG. 2C is a high resolution transmission electron microscopy (HRTEM)image of the BTO nanorods and the inset is a selected area electrondiffraction (SAED) pattern of the BTO nanorods;

FIG. 2D. is an fast Fourier transformation high resolution transmissionelectron microscopy (FFT-HRTEM) image of the BTO;

FIG. 3 is a schematic representation with SEM images of the BTO showingthe morphology change at various calcination temperatures;

FIG. 4 is a comparison of the diffuse reflectance (DR) measurements ofcommercial (a) TiO₂ (P-25) with (b) BTO nanorods. The inset showspictures of the (a) TiO₂ (P-25) with (b) BTO;

FIG. 5 is a schematic representation of band edges of BTO pyrochlorenanorods and TiO₂ from diffuse reflectant (DR) UV-vis and densityfunctional theory (DFT) calculations;

FIG. 6 is a photochemical reactor set-up used to study the hydrogenevolution reaction;

FIG. 7 is a graph of time based hydrogen evolution data following UV-visillumination of a methanol water solution (1:5 ratio) containing thephotocatalysts; and

FIG. 8 is a graph of the photodegradation of methyl orange (MO) in thepresence of visible light (λ≧405 nm) illumination and photocatalystfilms prepared on indium tin oxide (ITO) covered glass slides using (a)Degussa P-25 TiO₂ and (b) BTO nanorods.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In accordance with embodiments of the invention, a template-free reversemicelle (RM) based method is used to synthesize pyrochlorenanostructures having photocatalytic activity. For purposes herein, apyrochlore is a chemical compound characterized by the general formulaA₂B₂O₇, where A and B can be rare-earth or transition metal species, asfurther discussed in more detail below, and O is oxygen.

With reference to FIG. 1A, a general method of synthesizing pyrochlorenanostructures having the general formula A₂B₂O₇ includes separatelymixing together a first acid stabilized aqueous solution includingpyrochlore precursor A and a second acid stabilized aqueous solutionincluding pyrochlore precursor B with an organic solvent mixtureincluding a surfactant and an organic solvent to form an acid stabilizedoil-in-water emulsion. Equimolar portions of the first and second acidstabilized aqueous solutions then are mixed together to form a mixedacid stabilized oil-in-water emulsion of the pyrochlore precursor A andthe pyrochlore precursor B. The mixed acid stabilized oil-in-wateremulsion is next mixed with a base to raise the pH from acidic toalkaline. According to one embodiment, the base may be a basicoil-in-water emulsion, which is prepared by mixing together a basicaqueous solution with an organic solvent mixture including a surfactantand an organic solvent. Mixing the basic oil-in-water emulsion with themixed acid stabilized oil-in-water emulsion produces a precipitateincluding the pyrochlore precursors A and B. In other words, the mixedacid stabilized oil-in-water emulsion is treated with the basicoil-in-water emulsion to raise the pH from acidic to alkaline (e.g.,pH >7), which promotes and/or induces the formation of the precipitateincluding hydroxide salts containing A and B in the desired equimolarratio. The precipitate is isolated and dried. Upon heating theprecipitate to elevated temperatures in the presence of oxygen, apyrochlore nanostructure, such as a pyrochlore nanorod, is formedwithout the use of a template.

According to embodiments of the present invention, suitable examples ofA of pyrochlore precursor A can include the following elements: bismuth(Bi), calcium (Ca), strontium (Sr), yttrium (Y), barium (Ba), lanthanum(La), and the like, and combinations thereof. Suitable examples of B ofpyrochlore precursor B can include the following elements: titanium(Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), nickel(Ni), zirconium (Zr), tin (Sn), hafnium (Hf), tantalum (Ta), tungsten(W), and the like, and combinations thereof.

According to embodiments of the present invention, suitable acidsinclude hydrochloric acid (HCl), nitric acid (HNO₃), acetic acid(CH₃CO₂H), sulfurous acid (H₂SO₃), formic acid (HCO₂H), perchloric acid(HClO₄), and the like. Generally, the selected acid should facilitatedissolution of the pyrochlore precursors A and B.

According to embodiments of the present invention, the organic solventmixture includes a surfactant, which generally can be an anionic,nonionic, or cationic surfactant. Anionic surfactants can be based onsulfate, sulfonate, phosphinate, or carboxylate anions. Exemplaryanionic surfactants include, but are not limited to, perfluorooctanoate(PFOA or PFO), perfluorooctanesulfonate (PFOS), sodium dodecyl sulfate(SDS), ammonium lauryl sulfate, and other alkyl sulfate salts, sodiumlaureth sulfate (also known as sodium lauryl ether sulfate (SLES)),sodium dodecyl benzene sulfonate (SDBS), alkyl benzene sulfonate, soaps,or fatty acid salts, sodium di(n-octyl) phosphinate, and the like.Nonionic surfactants can include poly(oxyethylene)(4) lauryl ether(Brij® 30), sorbitan monolaurate (Span® 80), and the like. And cationicsurfactants can include tetradecyl dimethyl benzyl ammonium chloride(TDMBAC), cetyltrimethylammonium bromide (CTMAB),tetradecyltrimethylammonium bromide (Triton® N-42), and the like. Theorganic solvent may be, for example, an alkane such as iso-octane,heptane, n-decane, cyclohexane, toluene, decaline, dodecane, and thelike; an alkanol such as n-butanol to n-dodecanol, and the like; or analkanone such as cyclopentanone, cyclohexanone, cycloheptanone, and thelike.

According to embodiments of the present invention, the base includesammonium hydroxide (NH₄OH), potassium hydroxide (KOH), sodium hydroxide(NaOH), ammonia (NH₃) gas, and the like. The base may be dissolved inwater to provide the basic aqueous solution.

With respect to the ratio between the water of the first and second acidstabilized aqueous solutions and the surfactant solution, the ratio mayrange from 4 to 40 v/v. In another example, the ratio may range from 10to 20 v/v. In yet another example, the water to surfactant solutionratio is about 15 v/v. Depending upon the ratio, different sizes andshapes of the resulting hydroxide precipitate may be obtained. As such,a method of controlling the size and shape of the hydroxide precipitateof pyrochlore precursors A and B includes controlling the water tosurfactant solution ratio of the acid stabilized oil-in-water emulsionpyrochlore precursors and/or the mixed acid stabilized oil-in-wateremulsion pyrochlore precursors.

Similarly, controlling the ratio of the water to surfactant solution forthe base (e.g., NH₄OH) also facilitates control of the size and shape ofthe hydroxide precipitate of pyrochlore precursors A and B. For example,the ratio may range from 4 to 40 v/v. In another example, the ratio mayrange from 10 to 20 v/v. In yet another example, the water to surfactantratio of the base is about 15 v/v.

The independent mixing together of the acid stabilized aqueous solutionsof pyrochlore precursors A and B with the organic solvent mixture toform separate oil-in-water emulsions may be accomplished by any suitablemeans known to those skilled in the art. According to one embodiment,mixing can include ultrasonicating the oil-in-water emulsions. Withoutbeing bound by any particular theory, it is believed thatultrasonication of each of the solutions initiates the formation ofcolloid solutions of a reverse micelle. Ultrasound mixing may also beused when the two acid stabilized solutions are combined together, andupon subsequent treatment thereof with the base.

The precipitate may be collected by the customary methods such asfiltration or centrifugation. The precipitate may be washed with DIwater and dried to substantially remove volatiles. For example, theprecipitate may be heated overnight in air at 100° C. After drying, theprecipitate is subjected to further heating at various temperatures toaffect carbonization of residual organics and calcinations so as to formthe desired pyrochlore nanostructures, such as pyrochlore nanorods.

The following descriptions serve to provide exemplary embodiments of theinvention.

With specific reference now to FIG. 1B, an acid stabilized equimolaraqueous solution of a bismuth precursor, i.e., precursor A of A₂B₂O₇,and an acid stabilized equimolar aqueous solution of a titaniumprecursor, i.e., precursor B of A₂B₂O₇, are separately prepared. Inparticular, each of a 0.1M bismuth chloride in 1M HCl solution (acidstabilized aqueous solution of pyrochlore precursor A) and a 0.1Mtitanium isopropoxide in 1M HCl solution (acid stabilized aqueoussolution of pyrochlore precursor B) is prepared. Other bismuth salts maybe utilized in place of the BiCl₃ salt including bismuth nitrate,bismuth fluoride, bismuth iodide, bismuth oxychloride, and the like. Inaddition, other titanium precursors may utilized including titanium(IV)(triethanolaminato) isopropoxide, titanium(IV) bis(ethylacetoacetato)diisopropoxide, titanium(IV) oxyacetylacetonate,titanium(IV) oxychloride-hydrochloric acid solution, titanium(IV)oxysulfate-sulfuric acid solution, titanium(IV) tert-butoxide, and thelike. And HCl may be replaced by other acids including, for example,nitric acid (HNO₃), acetic acid (CH₃CO₂H), sulfurous acid (H₂SO₃),formic acid (HCO₂H), perchloric acid (HClO₄), and the like.

Next, each of the acid stabilized oil-in-water emulsions of bismuth andtitanium can be prepared by using a surfactant such as sodiumbis(2-ethylhexyl) sulphosuccinate (AOT) in iso-octane. According to oneexample, a 0.1M AOT in iso-octane solution is prepared. The bismuthsolution and the titanium solution are added drop wise to its ownAOT-iso-octane solution.

According to this example, 75 ml of 0.1M Ti salt solution is added to 5ml of 0.1M AOT and 75 ml of 0.1 M Bi salt solution added to 5 ml of 0.1MAOT, which provides a water to surfactant solution ratio of 15 v/v. One(1) hour of ultrasonication of each of the combined solutions initiatesthe formation of white colloid solutions of the reverse micelle.Ultrasound was used for vigorous mixing and to ensure the transport ofthe materials across the reverse micelles. Equal amounts of each of thebismuth and titanium white colloid solutions then are added togetherdrop wise and ultrasonicated for one (1) hour.

A 1M ammonium hydroxide in 0.1M AOT-iso-octane solution (oil-in-wateremulsion of base) is also separately prepared and sonicated for 1 hr toinitiate formation of a white colloid solution of the reverse micelle.In particular, 75 ml of 1M NH₄OH (basic aqueous solution) is added to 5ml of 0.1M AOT (surfactant solution in organic solvent), which alsoprovides a water to surfactant solution ratio of 15 v/v. Aside fromNH₄OH, KOH, NaOH, NH₃ gas, and the like may be used to provide a basicsolution. This solution is added drop wise to the combined acidstabilized bismuth and titanium oil-in-water emulsions to increase thepH from acidic to alkaline (pH=9.5), thereby causing a white precipitateto form. The precipitate includes the bismuth salt and titanium saltfrom the reverse micelle configuration. The precipitate is collected,washed with DI water, and dried by heating overnight in air at 100° C.to form a white powder of the hydroxide of Bi—Ti in the fixedcombination of ratio from the synthesis.

With further reference now to FIGS. 2A-2D, and FIG. 3, overnight dryingwas followed by carbonization in nitrogen at 350° C. and furtherfollowed by calcination of the powder in oxygen at various temperatures.Calcination in the presence of oxygen at 500° C. leads to the formationof spherical nanoparticles with a diameter of about 20 nm. When thenanoparticles are further calcined at 650° C., they rearrange to form arod-shaped morphology having good uniformity, with narrow sizedistribution, and an aspect ratio of about 1:10 (FIG. 2A).

FIG. 2B shows the X-ray diffraction (XRD) pattern of the nanorods formedby calcination at 650° C. The nanorods are identified as crystallinecubic Bi₂Ti₂O₇ with a space group of Fd3m and the lattice parametersa=b=c=20.68 Å (JCPDS #32-0118). X-ray diffraction (XRD) was performedusing a Philips APD 1740 system equipped with a graphite crystalmonochromator and CuKα radiation (λ=1.54 Å) between the ranges of 20° to80°=2θ. XRD was carried out using specially obtained polished quartz cut6° from (0001) crystal plane with a cavity of dimension 3 mm diameterand 1 mm depth (Gem Dugout X-ray Diffraction Products).

FIG. 2C shows a high resolution transmission electron microscopy (HRTEM)image of the BTO nanorods. The rods have a diameter of about 40-50 nmand a length from about 400-500 nm. Selected area electron diffractionpattern (SAED) shows that the nanorods have a (4 4 4) orientation. TheEnergy-Dispersive Spectroscopy (EDS) spectrum of BTO nanorods shows thepresence of Bi, Ti and O. Bi and Ti have identical atomic concentration.

FIG. 2D shows a high resolution Fast Fourier Transformation (FFT)-HRETMimage of the BTO pyrochlore nanorods with the (4 4 4) orientation aswell. From the HRTEM, EDS, and SAED analysis, it can be concluded thatthe nanorods are pure crystalline pyrochlore BTO. Further, the BTOnanoparticles are about 10 times smaller in size compared to thesmallest particles reported previously.

The formation of BTO nanorods can be hypothesized as a solid-phasereaction with a three-step mechanism as follows.

(1) Transport of reactants to the reaction zone: Hydroxides of bismuthand titanium precipitated in the microemulsion constitute the reactionzone. The formation of respective oxides is facilitated by calcinationin oxygen at 500° C. The presence of Bi₂O₃ and TiO₂ is evident from XRD.Bi₂O₃ present in the reaction zone can become more reactive by the phasetransformation from monoclinic to cubic with increase in temperature.This reactive Bi₂O₃ leads to the formation of solid solution with TiO₂at 650° C., as is evident from the phase diagram of Bi₂O₃ and TiO₂.

(2) Nucleation of new phase: The solid solution formed in the reactionzone leads to the creation of a new phase in the pyrochlore crystalstructure, which is also evident from the XRD data (FIG. 2B).

(3) Growth of the new phase: Variation in growth with temperature can beattributed to the differences in the free energy of formation of theBTO. A preference for growth along the (1 1 1) direction is noted fromXRD. In this orientation, the pyrochlore has a thermodynamically mostfavorable low free energy of formation. The growth of the crystallinenanorods in the pyrochlore phase is confirmed from the XRD (FIG. 2B) andFFT-HRTEM lattice pattern (FIG. 2D). A “d-spacing” value at 2.98 Åcorresponding to the high intensity (4 4 4) is noted from the XRDanalysis. This represents growth along the (1 1 1) direction aftercalcination at 650° C.

The nanorods formed at 650° C. in the pyrochlore phase are thermallystable up to 750° C. Increasing the temperature beyond 750° C. causesthe nanorods to fuse together to form an aggregate as seen from FIG. 3.A conventional solid state reaction was also performed by taking aphysical mixture of Bi₂O₃ and TiO₂ powders in stoichiometric ratio andthen calcining in oxygen at 650° C. for 6 h. The resulting material didnot yield a pyrochlore phase. This indicates the existence of asynergism between the nanoparticle nucleation in the RM environment,growth, and the calcination temperature. These parameters appear to playa role in the evolution of the particle size, shape, and pyrochlorephase formation.

With reference to FIG. 4, optical measurements were conducted to examinethe visible light response of BTO nanorods formed at 650° C. TheDRUV-vis spectra of BTO nanorods display a marked red shift in the onsetabsorbance by 48 nm than TiO₂ (commercial Degussa P-25). The absorbancein the visible light is not at a significant loss in UV absorbance whichis typically true for transition metal doped TiO₂. The inset of FIG. 4shows the color of BTO as pale yellow.

With reference to FIG. 5, Density Functional Theory (DFT) calculationswere carried out to determine the Partial Density of States (PDOS) ofBTO. Plane wave based Density Functional Theory (DFT) calculations wereuse to analyze the electronic property of the BTO. CASTEP programpresent in the Materials Studio® package supplied by Accelrys® was usedfor modeling. Plane wave functionalized ultrasoft pseudopotentials wereused with a kinetic energy cutoff of 300 eV. The Generalized GradientApproximation (GGA) with Perdew-Wang exchange and correlation functional(PW91) has been adopted. BTO with a pyrochlore crystal structure hasbeen obtained from ICSD (ICSD#413013). All the BTO properties have beencalculated by utilizing a primitive unit cell of the BTO pyrochlorecrystal structure. To compare the properties of BTO, anatase TiO₂(ISCD#. 82080) has been considered as a standard. A reduction in theband gap of about 0.4 eV compared to anatase TiO₂ crystal was evidentfrom PDOS plots determined from the DFT calculation of BTO pyrochloreand TiO₂ anatase. This band gap reduction and shift of valence band isdue to the contribution of 6s electrons from Bi³⁺. The schematicrepresentation of band positions of BTO has also been plotted againstthe water redox potential by using band gap calculated from DRUV-visdata. Thus, the DFT calculations complement the experimental bandgapestimates.

With respect to Table 1 below, the applicability of the BTO nanorods asa potential photocatalyst for splitting water to produce hydrogen wasexamined. In particular, photo catalytic hydrogen generation experimentswere performed in a slurry type photochemical reactor (See FIG. 6)supplied by Ace Glass, Vineland, N.J. The part number of the completereactor setup is 7840-340. The major parts of the setup, as shown in theFIG. 6, include a reactor 10, a concentric quartz tube 14 (Ace Glasspart #7854) that is adapted to house a lamp 18 and fits inside thereactor 10. The concentric quartz tube 14 includes a cooling water inlet22 and a cooling water outlet 26 and is designed to circulate water formaintaining the temperature of the reaction mixture, where the waterflows between the lamp and the contents of the reactor. The lamp 18 isconnected to a power source 30 by an electrical connector 34. Thereactor 10 further includes a reactant addition port 38, which permitsthe introduction of reactants into the reactor 10, and a gas outlet port42, which permits sampling reaction gases. The reactor 10 may furtherinclude additional features such as a temperature measuring port 46 andthermometer 50.

In this experiment, 150 mg of the photocatalyst (BTO as well as TiO₂)was weighed and introduced into the reactor. 300 ml of methanol-watermixture was prepared in the ratio of 1:5. The reaction mixture wasbubbled with nitrogen for 30 min to remove dissolved oxygen beforeilluminating the system. The lamp was introduced into the annulus of thereactor at the location shown in FIG. 6. Cold tap water was circulatedaround the lamp to maintain the temperature of the reaction. The gasesevolved were collected through the outlet of the reactor. A gas samplingtube with a rubber cork was used to collect gas from the outlet. A gaschromatograph (GC) was used to identify the gases evolved. The GC wascalibrated using high purity (>99.995) hydrogen. The collected gas wasanalyzed using a HP 6890 GC system with a TCD detector. A precisionHamilton® needle was used to remove representative sample of the gasfrom the sampling tube and injected into the GC. A molecular sievecapillary column HP-PLOT 5 A, 30 m, 0.32 mm, 25 μm was used to determinethe concentration of hydrogen.

With reference to FIG. 7, as well as Table 1, the results show that bareBTO nanorods demonstrate almost twice the hydrogen evolution of TiO₂.All the experiments were performed using identical mass of BTO and TiO₂.The activity of the bare BTO can be further enhanced by suitablesubstitution of different elements, either at the A site or B site ofthe flexible A₂B₂O₇ structure. Alternately, the addition of aco-catalyst such as Pt or NiO can further enhance the hydrogengeneration activity.

TABLE 1 Comparison of the photocatalytic activity of stoichiometric BTOand commercial TiO₂ (Degussa P-25). Bandgap has been estimated usingDRUV-vis and DFT studies. MO degradation H₂ evolution Bandgap Catalyst(vis light λ ≧405 nm (UV-vis light) E_(g) (eV) TiO₂ <1% 140 ml 3.24 BTO14% 285 ml 2.88

The photocatalytic ability of the BTO under visible light (λ≧405 nm) wasalso examined by monitoring the degradation of a textile dye, MethylOrange. TiO₂ showed negligible photodegradation whereas BTO nanorodsshow 14% degradation during the same period (See Table 1). Thephotodegradation experiments were performed by coating thephotocatalysts (BTO power or Degussa P 25 TiO₂) on an indium tin oxide(ITO) covered glass slide. Before coating, the ITO glass slides werewashed thoroughly with DI water followed by ultrasonication iniso-propyl alcohol for 15 min. Conducting side of the cleaned ITO glassslides were coated with a solution prepared by mixing ethylene glycol(10 mL), ethanol (5 mL), polyvinylpyrrolidone (5 mg), and 10 mg ofphotocatalyst. The photocatalyst coated ITO glass slides were dried andannealing in oxygen for 3 h at 500° C. to remove any organic molecules.This makes a uniform film coating of the photocatalyst on ITO. A similarprocedure was adopted to coat DegussaP-25. The photoactivity of theDegussa P-25 film was compared with the photo degradation activity ofBTO films using 20 μM methyl orange in DI water as a test compound. Thesetup used for performing the photodegradation experiments is describedin detail in Subramanian et al, Ind. Eng. Chem. Res., 2006, 45, 2187;Sohn et al., Appl. Catal B: Environ. 2008, 84, 372; and/or Kar et al.,Environ. Sci. Technol. 2009, 43, 32, which are expressly incorporated byreference herein in their entirety.

A 405 nm cutoff filter purchased from Newport Corporation was used toexamine the effects of visible light illumination. Photocatalyticdegradation of MO as a function of time is shown in FIG. 8. Thisconfirms that the BTO nanorods demonstrate photoactivity in visiblelight.

A simple and robust template-free RM method to synthesize highlycrystalline stoichiometric pyrochlore nanostructures, e.g., bismuthtitanate nanorods, is disclosed herein. The BTO nanorods demonstrateimproved photocatalytic hydrogen generation as compared to, for example,commercial Degussa P-25. The BTO also shows visible light activity. Themethod may be used for preparing pyrochlores nanostructures for wide arange of applications including catalysis, electronics, and sensors.

While the present invention has been illustrated by a description ofvarious embodiments and while these embodiments have been described inconsiderable detail, it is not the intention of the applicant torestrict or in any way limit the scope of the appended claims to suchdetail. The methods described herein present a new way to synthesize notjust BTO but a family of compounds belonging to pyrochlores. Forexample, while discussion of the reverse micelle (RM) basedtemplate-free synthesis method has centered on synthesizingstoichiometric BTO pyrochlore nanorods, it should be understood that thebismuth and/or titanium may be replaced with other suitable materials,as desired. In other words, the A and/or B in A₂B₂O₇ can be somethingother than bismuth or titanium, as discussed herein. In addition, whileBTO in stoichiometric pyrochlore (Bi₂Ti₂O₇) crystal structure showspromising photocatalytic activity, pyrochlores allow internalflexibility for different charge balanced combinations as wellincluding, for example, A₂ ³⁺B₂ ⁴⁺O₇, A₂ ²⁺B₂ ⁵⁺O₇, and A₂ ¹⁺B₂ ⁶⁺O₇.Additional advantages and modifications will readily appear to thoseskilled in the art. Thus, the invention in its broader aspects istherefore not limited to the specific details, representative apparatusand method, and illustrative example shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of applicant's general inventive concept.

1. A method of synthesizing a pyrochlore nanostructure having thegeneral formula A₂B₂O₇, where A and B are rare-earth or transition metalspecies and O is oxygen, the method comprising: separately mixingtogether a first acid stabilized aqueous solution including a pyrochloreprecursor A and a second acid stabilized aqueous solution including apyrochlore precursor B with an organic solvent mixture comprising asurfactant and an organic solvent to form a first acid stabilizedoil-in-water emulsion of precursor A and a second acid stabilizedoil-in-water emulsion of precursor B; mixing together equimolar portionsof the first and second acid stabilized oil-in-water emulsions to form amixed acid stabilized oil-in-water emulsion comprising the pyrochloreprecursor A and the pyrochlore precursor B; treating the mixed acidstabilized oil-in-water emulsion with a base to increase the pH fromacidic to alkaline to produce a precipitate including the pyrochloreprecursors A and B; isolating the precipitate; and calcining theprecipitate in the presence of oxygen to form the pyrochlorenanostructure, wherein the method of synthesizing the pyrochlorenanostructure is template-free.
 2. The method of claim 1, wherein B ofthe pyrochlore precursor B is titanium (Ti), vanadium (V), chromium(Cr), manganese (Mn), iron (Fe), nickel (Ni), zirconium (Zr), tin (Sn),hafnium (Hf), tantalum (Ta), tungsten (W), or combinations thereof. 3.The method of claim 1, wherein A of the pyrochlore precursor A isbismuth (Bi), calcium (Ca), strontium (Sr), yttrium (Y), barium (Ba),lanthanum (La), or combinations thereof.
 4. The method of claim 1,wherein the first and second acid stabilized solutions each include anacid independently selected from hydrochloric acid (HCl), nitric acid(HNO₃), acetic acid (CH₃CO₂H), sulfurous acid (H₂SO₃), formic acid(HCO₂H), perchloric acid (HClO₄), or combinations thereof.
 5. The methodof claim 1, wherein the surfactant is selected from an anionicsurfactant comprising a sulfate, a sulfonate, a phosphinate, or acarboxylate moiety; a cationic surfactant comprising an ammonium moiety;or a nonionic surfactant.
 6. The method of claim 1, wherein the organicsolvent is selected from an alkane, an alkanol, an alkanone, orcombinations thereof.
 7. The method of claim 1, wherein calcining isperformed at a temperature greater than 350° C. and less than 900° C. 8.The method of claim 7, wherein calcining is performed at a temperaturegreater than 500° C.
 9. The method of claim 1 wherein the pyrochlorenanostructure is a pyrochlore nanorod.
 10. The method of claim 9 whereinthe pyrochlore nanorod is a bismuth titanate (Bi₂Ti₂O₇) pyrochlorenanorod.
 11. The method of claim 1 wherein the pyrochlore nanostructurehas photocatalytic activity.
 12. The method of claim 1 wherein a ratioof water to organic solvent mixture of the first and second acidstabilized aqueous solutions is from 4 to 40 v/v.
 13. A pyrochlorenanostructure having the general formula A₂B₂O₇, wherein A is bismuth(Bi), calcium (Ca), strontium (Sr), yttrium (Y), barium (Ba), lanthanum(La), or combinations thereof; B is titanium (Ti), vanadium (V),chromium (Cr), manganese (Mn), iron (Fe), nickel (Ni), zirconium (Zr),tin (Sn), hafnium (Hf), tantalum (Ta), tungsten (W), or combinationsthereof; and O is oxygen; and wherein the nanostructure has a sphericalor rod shape.
 14. The pyrochlore nanostructure of claim 13, wherein thepyrochlore nanostructure is sphere-shaped and has a diameter of about 20nm.
 15. The pyrochlore nanostructure of claim 13, wherein the pyrochlorenanostructure is rod-shaped.
 16. The pyrochlore nanostructure of claim15, wherein the rod-shape pyrochlore nanostructure has an aspect ratioof about
 10. 17. The pyrochlore nanostructure of claim 15, wherein therod-shape pyrochlore nanostructure has a diameter of about 40 nm toabout 50 nm, and a length of about 400 nm to about 500 nm.
 18. Thepyrochlore nanostructure of claim 13 having photocatalytic activity. 19.The pyrochlore nanostructure of claim 13 wherein A is bismuth (Bi); B istitanium (Ti); and wherein the nanostructure is rod-shaped.